Alternating current driven load circuit



Ullllefl Dial Louis A. mnunl Trenton. NJ.

Mar. 2 l, 1969 Dec. 29, 1970 RCA Corporation a corporation of Delaware Inventor Appl. No. Filed Patented Assignee ALTERNATING CURRENT DRIVEN LOAD CIRCUIT 4 Claims, 2 Drawing Figs.

U.S. Cl 307/38;

Int. Cl H02j 3/14 Field of Search 350/ l 50,

160(lnquired): 307/253 (inquired), 31 l 38, 39. 41 340/324.l (inquired): 3l5/l49 [56] References Cited UNITED STATES PATENTS 3.299.284 1/1967 Hough 307/253 3,322,485 5/1967 Williams 350/160 Primary Examiner-Robert K. Schaefer Assistant Examiner-H. J. Hohauser AIl0rne \'H. Christoffersen LIQUID CIZYS'ML QR mam/3a PATENFED 05229 I976 m uouro 6:2Ys1% 5 m m m INVENTOI? LOUIS A. Znuom 47' TORHEY ALTERNATING CURRENT DRIVEN LOAD CIRCUIT BACKGROUND OF THE INVENTION Liquid crystals such as'those of the nematic type which operate on the principle of dynamic scattering have optical properties which make them suitable for alpha-numeric, television and other types of displays, light shutters and so on. Such crystals, when in an unexcited state, appear to be relatively transparent and, when in an excited state, reflect the major portion of the light reaching the crystal. It has been found that when a direct voltage is employed to produce the electrical field necessary to excite the crystal, electrochemical reactions occur which degrade the performance of the liquid crystal. In particular, the life of the crystal is very much shortened.

The object of the present invention is to provide an improved switch circuit for permitting alternating voltage operation of liquid crystals.

BRIEF SUMMARY OF THE INVENTION The circuit of the invention includes two terminals for an alternating current power supply and a series circuit connected between these terminals comprising the emitter-to-collector path of a transistor. a capacitor and a load which may be liquid crystal element. The circuit also includes means for placing the transistor in conducting and nonconducting conditions comprising means for applying a signal to the base of the transistor.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a preferred form of the present invention; and

FIG. 2 is an equivalent circuit of the circuit of FIG. 1.

DETAILED DESCRIPTION The circuit of FIG. 1 includes a common, alternating voltage power supply such as a 60-cycle source connected to the primary winding 12 of a power transformer 14. The secondary winding 16 of the transformer is connected to all of the liquid crystal elements of a display, such as an alpha-numeric display. For purposes of the present discussion, only two of the elements 18 and 20 are illustrated.

As the switch circuit for each liquid crystal element is identical, only one such circuit is described. It includes the emitter 22 to collector 24 path of a transistor 26 in series with a capacitor 28. A source of turn-on voltage is connected via terminal 30 and resistor 32 to the base 34 of the transistor 26. The transistor may be of the NPN type, as shown, in which case the turn-on voltage required is relatively positive. For example, the turn-on voltage may be a pulse starting at 0 volts which reaches an amplitude of +4 volts.

The operation of the circuit of FIG. 1 may be better understood by referring to FIG. 2. Here, the transistor 26 is shown in equivalent circuit form as two diodes 36 and 38 connected anode to anode. Diode 38 is the emitter-to-base diode and diode 36 is the base-to-collector diode. When the transistor 26 is turned on, it is forward biased sufficiently that the transistor operates in saturation. Current therefore flows from the secondary winding 16 of the transformer through the liquid crystal cell and the emitter-to-collector path of the transistor during both the positive and negative portions of the sine wave 40 driving the crystal. This current causes dynamic scattering of light to occur in the crystal.

When it is desired to turn the liquid crystal off, the base 34 is returned to ground as shown in FIG. 2. As the emitter and base are now at the same potential. the emitter-to-base diode 38 does not conduct. However, it would appear, at first consideration, that each time the alternating voltage driving signal 40 of FIG. 2 went negative, the base-to-collector diode 36 would be driven into conduction and the liquid crystal cell 18 would, rather than being turned ofi, exhibit dynamic scattermg.

In practice, the above actually does occur but only for a few cycles. By that time, a charge develops on the capacitor 28. as shown, that is, the capacitor plate connected to the cathode of the base-to-collector diode 36 becomes sufficiently positive with respect to ground that diode 36 becomes reverse biased and no longer can conduct. The switch 26 therefore opens (the transistor becomes nonconducting) and the power supply voltage develops mainly across the relatively high impedance of the transistor. The voltage and electric field across the liquid crystal reduces correspondingly to a value sufficiently low that the crystal is turned off (is returned to its quiescent state in which it appears relatively transparent).

It is found, in practice, that the switch circuit described provides sufficient isolation among the liquid crystal cells of a display, such as an alpha-numeric display, that a single power supply may be employed for all liquid crystal cells and any one or more cells may be individually turned on without affecting the state of the remaining cells.

In one practical circuit, the following values of circuit elements and circuit parameters were employed. These values, of course, are intended merely to be illustrated and are not to be taken as limiting.

Resistor 32 ol1ms 3900 Capacitor 28 microfarads 0. 001 Transistor 26, type RCA 40, 084 The value of alternating voltage appearing across secondary winding 16, peak-topeak T volts While in the circuit given by way of example, NPN transistors are employed, it is to be appreciated, of course, that PNP transistors may be used instead provided the proper polarity of turn-on voltage is employed. Also, while the liquid crystal discussed is one of the type operating on the principle of dynamic scattering, the invention is equally applicable to other types of liquid crystals.

In the claims which follow, the expression, series connected circuit is employed in the strict sense as set forth in the International Dictionary of Physics and Electronics, Van Nostrand, 1956, pages 817 and 818. As explained there, network elements are said to be series connected if they carry a common current. As also stated there, two terminal elements are connected in series when they form a path between two nodes of a network such that only elements of this path and no other elements terminate at intermediate nodes along the path.

I claim:

1. In combination:

two terminals for an alternating current power supply;

a transistor having a base, an emitter and a collector, said emitter and collector defining the ends of a conduction path through the transistor, said path being connected at one end to one of said terminals;

a series connected circuit comprising a capacitor in series with a load, said series connected circuit being connected at one end to the other of said terminals and at its other end to the other end of said path; and

means for placing said transistor in conducting and nonconducting conditions comprising means for applying a signal to the base of said transistor.

2. In the combination set forth in claim 1, said load comprising a liquid crystal cell.

3. In the combination set forth in claim 2, said transistor being connected at its collector electrode to one terminal of said capacitor and said capacitor being connected at its other terminal to said liquid crystal cell.

4. In combination:

two terminals for an alternating current supply;

a plurality of circuits individually connected between said terminals, each said circuit comprising:

a bipolar transistor having a base, emitter, and a collector, said emitter and collector defining the ends of a conduction path through the transistor, said path being the other of said terminals and at its other end to the connected at one end to one of said terminals; and other end of said path; and a sqles collne'cted Circuit Comprlsmg a p' ries means for selectively placing said transistors in conducting with a liquid crvstal cell of the nematic type which J and nonconductmg condition.

operates on the principle of dynamic scattering, said se 5 ries connected circuit being connected at one end to 

